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  <div class="section" id="library-conventions">
<h1>5. Library conventions</h1>
<div class="section" id="error-handling">
<h2>5.1. Error handling</h2>
<div class="section" id="return-status">
<h3>5.1.1. Return status</h3>
<p>Almost all functions return a status indication of type <a class="reference internal" href="../api/library/status.html#c.psa_status_t" title="psa_status_t"><code class="xref any c c-type docutils literal notranslate"><span class="pre">psa_status_t</span></code></a>. This
is an enumeration of integer values, with <code class="docutils literal notranslate"><span class="pre">0</span></code> (<a class="reference internal" href="../api/library/status.html#c.PSA_SUCCESS" title="PSA_SUCCESS"><code class="xref any c c-macro docutils literal notranslate"><span class="pre">PSA_SUCCESS</span></code></a>) indicating
successful operation and other values indicating errors. The exceptions are
functions which only access objects that are intended to be implemented as
simple data structures. Such functions cannot fail and either return
<code class="docutils literal notranslate"><span class="pre">void</span></code> or a data value.</p>
<p>Unless specified otherwise, if multiple error conditions apply, an
implementation is free to return any of the applicable error codes. The choice
of error code is considered an implementation quality issue. Different
implementations can make different choices, for example to favor code size over
ease of debugging or vice versa.</p>
<p>If the behavior is undefined, for example, if a function receives an invalid
pointer as a parameter, this specification makes no guarantee that the function
will return an error. Implementations are encouraged to return an error or halt
the application in a manner that is appropriate for the platform if the
undefined behavior condition can be detected. However, application developers need to be aware that undefined behavior conditions cannot be detected in general.</p>
</div>
<div class="section" id="behavior-on-error">
<h3>5.1.2. Behavior on error</h3>
<p>All function calls must be implemented atomically:</p>
<ul class="simple">
<li><p>When a function returns a type other than <a class="reference internal" href="../api/library/status.html#c.psa_status_t" title="psa_status_t"><code class="xref any c c-type docutils literal notranslate"><span class="pre">psa_status_t</span></code></a>, the requested
action has been carried out.</p></li>
<li><p>When a function returns the status <a class="reference internal" href="../api/library/status.html#c.PSA_SUCCESS" title="PSA_SUCCESS"><code class="xref any c c-macro docutils literal notranslate"><span class="pre">PSA_SUCCESS</span></code></a>, the requested action has
been carried out.</p></li>
<li><p>When a function returns another status of type <a class="reference internal" href="../api/library/status.html#c.psa_status_t" title="psa_status_t"><code class="xref any c c-type docutils literal notranslate"><span class="pre">psa_status_t</span></code></a>, no action
has been carried out. The content of the output parameters is undefined, but
otherwise the state of the system has not changed, except as described below.</p></li>
</ul>
<p>In general, functions that modify the system state, for example, creating or
destroying a key, must leave the system state unchanged if they return an error
code. There are specific conditions that can result in different behavior:</p>
<ul class="simple">
<li><p>The status <a class="reference internal" href="../api/library/status.html#c.PSA_ERROR_BAD_STATE" title="PSA_ERROR_BAD_STATE"><code class="xref any c c-macro docutils literal notranslate"><span class="pre">PSA_ERROR_BAD_STATE</span></code></a> indicates that a parameter was not in a
valid state for the requested action. This parameter might have been modified
by the call and is now in an undefined state. The only valid action on an
object in an undefined state is to abort it with the appropriate
<code class="docutils literal notranslate"><span class="pre">psa_abort_xxx()</span></code> function.</p></li>
<li><p>The status <a class="reference internal" href="../api/library/status.html#c.PSA_ERROR_INSUFFICIENT_DATA" title="PSA_ERROR_INSUFFICIENT_DATA"><code class="xref any c c-macro docutils literal notranslate"><span class="pre">PSA_ERROR_INSUFFICIENT_DATA</span></code></a> indicates that a key
derivation object has reached its maximum capacity. The key derivation
operation might have been modified by the call. Any further attempt to obtain
output from the key derivation operation will return
<a class="reference internal" href="../api/library/status.html#c.PSA_ERROR_INSUFFICIENT_DATA" title="PSA_ERROR_INSUFFICIENT_DATA"><code class="xref any c c-macro docutils literal notranslate"><span class="pre">PSA_ERROR_INSUFFICIENT_DATA</span></code></a>.</p></li>
<li><p>The status <a class="reference internal" href="../api/library/status.html#c.PSA_ERROR_COMMUNICATION_FAILURE" title="PSA_ERROR_COMMUNICATION_FAILURE"><code class="xref any c c-macro docutils literal notranslate"><span class="pre">PSA_ERROR_COMMUNICATION_FAILURE</span></code></a> indicates that the
communication between the application and the cryptoprocessor has broken
down. In this case, the cryptoprocessor must either finish the requested
action successfully, or interrupt the action and roll back the system to its
original state. Because it is often impossible to report the outcome to the
application after a communication failure, this specification does not
provide a way for the application to determine whether the action was
successful.</p></li>
<li><p>The statuses <a class="reference internal" href="../api/library/status.html#c.PSA_ERROR_STORAGE_FAILURE" title="PSA_ERROR_STORAGE_FAILURE"><code class="xref any c c-macro docutils literal notranslate"><span class="pre">PSA_ERROR_STORAGE_FAILURE</span></code></a>, <a class="reference internal" href="../api/library/status.html#c.PSA_ERROR_DATA_CORRUPT" title="PSA_ERROR_DATA_CORRUPT"><code class="xref any c c-macro docutils literal notranslate"><span class="pre">PSA_ERROR_DATA_CORRUPT</span></code></a>, <a class="reference internal" href="../api/library/status.html#c.PSA_ERROR_HARDWARE_FAILURE" title="PSA_ERROR_HARDWARE_FAILURE"><code class="xref any c c-macro docutils literal notranslate"><span class="pre">PSA_ERROR_HARDWARE_FAILURE</span></code></a>
and <a class="reference internal" href="../api/library/status.html#c.PSA_ERROR_CORRUPTION_DETECTED" title="PSA_ERROR_CORRUPTION_DETECTED"><code class="xref any c c-macro docutils literal notranslate"><span class="pre">PSA_ERROR_CORRUPTION_DETECTED</span></code></a> might indicate data corruption in the
system state. When a function returns one of these statuses, the system state
might have changed from its previous state before the function call, even
though the function call failed.</p></li>
<li><p>Some system states cannot be rolled back, for example, the internal state of
the random number generator or the content of access logs.</p></li>
</ul>
<p>Unless otherwise documented, the content of output parameters is not defined
when a function returns a status other than <a class="reference internal" href="../api/library/status.html#c.PSA_SUCCESS" title="PSA_SUCCESS"><code class="xref any c c-macro docutils literal notranslate"><span class="pre">PSA_SUCCESS</span></code></a>. It is recommended
that implementations set output parameters to safe defaults to avoid leaking
confidential data and limit risk, in case an application does not properly
handle all errors.</p>
</div>
</div>
<div class="section" id="parameter-conventions">
<h2>5.2. Parameter conventions</h2>
<div class="section" id="pointer-conventions">
<h3>5.2.1. Pointer conventions</h3>
<p>Unless explicitly stated in the documentation of a function, all pointers must
be valid pointers to an object of the specified type.</p>
<p>A parameter is considered a <strong>buffer</strong> if it points to an array of bytes. A
buffer parameter always has the type <code class="docutils literal notranslate"><span class="pre">uint8_t</span> <span class="pre">*</span></code> or <code class="docutils literal notranslate"><span class="pre">const</span> <span class="pre">uint8_t</span> <span class="pre">*</span></code>, and
always has an associated parameter indicating the size of the array. Note that a
parameter of type <code class="docutils literal notranslate"><span class="pre">void</span> <span class="pre">*</span></code> is never considered a buffer.</p>
<p>All parameters of pointer type must be valid non-null pointers, unless the
pointer is to a buffer of length <code class="docutils literal notranslate"><span class="pre">0</span></code> or the function’s documentation
explicitly describes the behavior when the pointer is null. Passing a null
pointer as a function parameter in other cases is expected to abort the caller
on implementations where this is the normal behavior for a null pointer
dereference.</p>
<p>Pointers to input parameters can be in read-only memory. Output parameters must
be in writable memory. Output parameters that are not buffers must also be
readable, and the implementation must be able to write to a non-buffer output
parameter and read back the same value, as explained in the
<a class="reference internal" href="#stability-of-parameters"><span class="secref">Stability of parameters</span></a> section.</p>
</div>
<div class="section" id="input-buffer-sizes">
<h3>5.2.2. Input buffer sizes</h3>
<p>For input buffers, the parameter convention is:</p>
<dl class="simple">
<dt><code class="docutils literal notranslate"><span class="pre">const</span> <span class="pre">uint8_t</span> <span class="pre">*foo</span></code></dt><dd><p>Pointer to the first byte of the data. The pointer
can be invalid if the buffer size is <code class="docutils literal notranslate"><span class="pre">0</span></code>.</p>
</dd>
<dt><code class="docutils literal notranslate"><span class="pre">size_t</span> <span class="pre">foo_length</span></code></dt><dd><p>Size of the buffer in bytes.</p>
</dd>
</dl>
<p>The interface never uses input-output buffers.</p>
</div>
<div class="section" id="output-buffer-sizes">
<h3>5.2.3. Output buffer sizes</h3>
<p>For output buffers, the parameter convention is:</p>
<dl class="simple">
<dt><code class="docutils literal notranslate"><span class="pre">uint8_t</span> <span class="pre">*foo</span></code></dt><dd><p>Pointer to the first byte of the data. The pointer can be
invalid if the buffer size is <code class="docutils literal notranslate"><span class="pre">0</span></code>.</p>
</dd>
<dt><code class="docutils literal notranslate"><span class="pre">size_t</span> <span class="pre">foo_size</span></code></dt><dd><p>The size of the buffer in bytes.</p>
</dd>
<dt><code class="docutils literal notranslate"><span class="pre">size_t</span> <span class="pre">*foo_length</span></code></dt><dd><p>On successful return, contains the length of the
output in bytes.</p>
</dd>
</dl>
<p>The content of the data buffer and of <code class="docutils literal notranslate"><span class="pre">*foo_length</span></code> on errors is unspecified,
unless explicitly mentioned in the function description. They might be unmodified
or set to a safe default. On successful completion, the content of the buffer
between the offsets <code class="docutils literal notranslate"><span class="pre">*foo_length</span></code> and <code class="docutils literal notranslate"><span class="pre">foo_size</span></code> is also unspecified.</p>
<p>Functions return <a class="reference internal" href="../api/library/status.html#c.PSA_ERROR_BUFFER_TOO_SMALL" title="PSA_ERROR_BUFFER_TOO_SMALL"><code class="xref any c c-macro docutils literal notranslate"><span class="pre">PSA_ERROR_BUFFER_TOO_SMALL</span></code></a> if the buffer size is
insufficient to carry out the requested operation. The interface defines macros
to calculate a sufficient buffer size for each operation that has an output
buffer. These macros return compile-time constants if their arguments are
compile-time constants, so they are suitable for static or stack allocation.
Refer to an individual function’s documentation for the associated output size
macro.</p>
<p>Some functions always return exactly as much data as the size of the output
buffer. In this case, the parameter convention changes to:</p>
<dl class="simple">
<dt><code class="docutils literal notranslate"><span class="pre">uint8_t</span> <span class="pre">*foo</span></code></dt><dd><p>Pointer to the first byte of the output. The pointer can be
invalid if the buffer size is <code class="docutils literal notranslate"><span class="pre">0</span></code>.</p>
</dd>
<dt><code class="docutils literal notranslate"><span class="pre">size_t</span> <span class="pre">foo_length</span></code></dt><dd><p>The number of bytes to return in <code class="docutils literal notranslate"><span class="pre">foo</span></code> if
successful.</p>
</dd>
</dl>
</div>
<div class="section" id="overlap-between-parameters">
<h3>5.2.4. Overlap between parameters</h3>
<p>Output parameters that are not buffers must not overlap with any input buffer or
with any other output parameter. Otherwise, the behavior is undefined.</p>
<p>Output buffers can overlap with input buffers. In this event, the implementation
must return the same result as if the buffers did not overlap. The
implementation must behave as if it had copied all the inputs into temporary
memory, as far as the result is concerned. However, it is possible that overlap
between parameters will affect the performance of a function call. Overlap might
also affect memory management security if the buffer is located in memory that
the caller shares with another security context, as described in the
<a class="reference internal" href="#stability-of-parameters"><span class="secref">Stability of parameters</span></a> section.</p>
</div>
<div class="section" id="stability-of-parameters">
<span id="id1"></span><h3>5.2.5. Stability of parameters</h3>
<p>In some environments, it is possible for the content of a parameter to change
while a function is executing. It might also be possible for the content of an
output parameter to be read before the function terminates. This can happen if
the application is multithreaded. In some implementations, memory can be shared
between security contexts, for example, between tasks in a multitasking
operating system, between a user land task and the kernel, or between the
Non-secure world and the Secure world of a trusted execution environment.</p>
<p>This section describes the assumptions that an implementation can make about
function parameters, and the guarantees that the implementation must provide
about how it accesses parameters.</p>
<p>Parameters that are not buffers are assumed to be under the caller’s full
control. In a shared memory environment, this means that the parameter must be
in memory that is exclusively accessible by the application. In a multithreaded
environment, this means that the parameter must not be modified during the
execution, and the value of an output parameter is undetermined until the
function returns. The implementation can read an input parameter that is not a
buffer multiple times and expect to read the same data. The implementation can
write to an output parameter that is not a buffer and expect to read back the
value that it last wrote. The implementation has the same permissions on buffers
that overlap with a buffer in the opposite direction.</p>
<p>In an environment with multiple threads or with shared memory, the
implementation carefully accesses non-overlapping buffer parameters in order to
prevent any security risk resulting from the content of the buffer being
modified or observed during the execution of the function. In an input buffer
that does not overlap with an output buffer, the implementation reads each byte
of the input once, at most. The implementation does not read from an output
buffer that does not overlap with an input buffer. Additionally, the
implementation does not write data to a non-overlapping output buffer if this
data is potentially confidential and the implementation has not yet verified
that outputting this data is authorized.</p>
<p>Unless otherwise specified, the implementation must not keep a reference to any
parameter once a function call has returned.</p>
</div>
</div>
<div class="section" id="key-types-and-algorithms">
<h2>5.3. Key types and algorithms</h2>
<p>Types of cryptographic keys and cryptographic algorithms are encoded separately.
Each is encoded by using an integral type: <a class="reference internal" href="../api/keys/types.html#c.psa_key_type_t" title="psa_key_type_t"><code class="xref any c c-type docutils literal notranslate"><span class="pre">psa_key_type_t</span></code></a> and
<a class="reference internal" href="../api/ops/algorithms.html#c.psa_algorithm_t" title="psa_algorithm_t"><code class="xref any c c-type docutils literal notranslate"><span class="pre">psa_algorithm_t</span></code></a>, respectively.</p>
<p>There is some overlap in the information conveyed by key types and algorithms.
Both types contain enough information, so that the meaning of an algorithm type
value does not depend on what type of key it is used with, and vice versa.
However, the particular instance of an algorithm might depend on the key type. For
example, the algorithm <a class="reference internal" href="../api/ops/aead.html#c.PSA_ALG_GCM" title="PSA_ALG_GCM"><code class="xref any c c-macro docutils literal notranslate"><span class="pre">PSA_ALG_GCM</span></code></a> can be instantiated as any AEAD algorithm
using the GCM mode over a block cipher. The underlying block cipher is
determined by the key type.</p>
<p>Key types do not encode the key size. For example, AES-128, AES-192 and AES-256
share a key type <a class="reference internal" href="../api/keys/types.html#c.PSA_KEY_TYPE_AES" title="PSA_KEY_TYPE_AES"><code class="xref any c c-macro docutils literal notranslate"><span class="pre">PSA_KEY_TYPE_AES</span></code></a>.</p>
<div class="section" id="structure-of-key-and-algorithm-types">
<h3>5.3.1. Structure of key and algorithm types</h3>
<p>Both types use a partial bitmask structure, which allows the analysis and
building of values from parts. However, the interface defines constants, so that
applications do not need to depend on the encoding, and an implementation might
only care about the encoding for code size optimization.</p>
<p>The encodings follows a few conventions:</p>
<ul class="simple">
<li><p>The highest bit is a vendor flag. Current and future versions of this
specification will only define values where this bit is clear.
Implementations that wish to define additional implementation-specific values
must use values where this bit is set, to avoid conflicts with future
versions of this specification.</p></li>
<li><p>The next few highest bits indicate the corresponding algorithm category:
hash, MAC, symmetric cipher, asymmetric encryption, and so on.</p></li>
<li><p>The following bits identify a family of algorithms in a category-dependent
manner.</p></li>
<li><p>In some categories and algorithm families, the lowest-order bits indicate a
variant in a systematic way. For example, algorithm families that are
parametrized around a hash function encode the hash in the 8 lowest bits.</p></li>
</ul>
</div>
</div>
<div class="section" id="concurrent-calls">
<span id="concurrency"></span><h2>5.4. Concurrent calls</h2>
<p>In some environments, an application can make calls to the PSA crypto API in
separate threads. In such an environment, <em>concurrent calls</em> are two or more
calls to the API whose execution can overlap in time.</p>
<p>Concurrent calls are performed correctly, as if the calls were executed in
sequence, provided that they obey the following constraints:</p>
<ul class="simple">
<li><p>There is no overlap between an output parameter of one call and an input or
output parameter of another call. Overlap between input parameters is
permitted.</p></li>
<li><p>A call to destroy a key must not overlap with a concurrent call to any of
the following functions:</p>
<ul>
<li><p>Any call where the same key identifier is a parameter to the call.</p></li>
<li><p>Any call in a multi-part operation, where the same key identifier was
used as a parameter to a previous step in the multi-part operation.</p></li>
</ul>
</li>
<li><p>Concurrent calls must not use the same operation object.</p></li>
</ul>
<p>If any of these constraints are violated, the behavior is undefined.</p>
<p>If the application modifies an input parameter while a function call is in
progress, the behavior is undefined.</p>
<p>Individual implementations can provide additional guarantees.</p>
</div>
</div>


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        <div class="sphinxsidebarwrapper"><h3><a href="../index.html"><b>PSA Crypto API</b></a></h3>
IHI 0086<br/>
Non-confidential<br/>
Version 1.0.1
<span style="color: red; font-weight: bold;"></span>
<ul>
<li class="toctree-l1"><a class="reference internal" href="../about.html">About this document</a></li>
</ul>
<ul class="current">
<li class="toctree-l1"><a class="reference internal" href="intro.html">1. Introduction</a></li>
<li class="toctree-l1"><a class="reference internal" href="goals.html">2. Design goals</a></li>
<li class="toctree-l1"><a class="reference internal" href="functionality.html">3. Functionality overview</a></li>
<li class="toctree-l1"><a class="reference internal" href="sample-arch.html">4. Sample architectures</a></li>
<li class="toctree-l1 current"><a class="current reference internal" href="#">5. Library conventions</a><ul>
<li class="toctree-l2"><a class="reference internal" href="#error-handling">5.1. Error handling</a><ul>
<li class="toctree-l3"><a class="reference internal" href="#return-status">5.1.1. Return status</a></li>
<li class="toctree-l3"><a class="reference internal" href="#behavior-on-error">5.1.2. Behavior on error</a></li>
</ul>
</li>
<li class="toctree-l2"><a class="reference internal" href="#parameter-conventions">5.2. Parameter conventions</a><ul>
<li class="toctree-l3"><a class="reference internal" href="#pointer-conventions">5.2.1. Pointer conventions</a></li>
<li class="toctree-l3"><a class="reference internal" href="#input-buffer-sizes">5.2.2. Input buffer sizes</a></li>
<li class="toctree-l3"><a class="reference internal" href="#output-buffer-sizes">5.2.3. Output buffer sizes</a></li>
<li class="toctree-l3"><a class="reference internal" href="#overlap-between-parameters">5.2.4. Overlap between parameters</a></li>
<li class="toctree-l3"><a class="reference internal" href="#stability-of-parameters">5.2.5. Stability of parameters</a></li>
</ul>
</li>
<li class="toctree-l2"><a class="reference internal" href="#key-types-and-algorithms">5.3. Key types and algorithms</a><ul>
<li class="toctree-l3"><a class="reference internal" href="#structure-of-key-and-algorithm-types">5.3.1. Structure of key and algorithm types</a></li>
</ul>
</li>
<li class="toctree-l2"><a class="reference internal" href="#concurrent-calls">5.4. Concurrent calls</a></li>
</ul>
</li>
<li class="toctree-l1"><a class="reference internal" href="implementation.html">6. Implementation considerations</a></li>
<li class="toctree-l1"><a class="reference internal" href="usage.html">7. Usage considerations</a></li>
<li class="toctree-l1"><a class="reference internal" href="../api/library/index.html">8. Library management reference</a></li>
<li class="toctree-l1"><a class="reference internal" href="../api/keys/index.html">9. Key management reference</a></li>
<li class="toctree-l1"><a class="reference internal" href="../api/ops/index.html">10. Cryptographic operation reference</a></li>
</ul>
<ul>
<li class="toctree-l1"><a class="reference internal" href="../appendix/example_header.html">Example header file</a></li>
<li class="toctree-l1"><a class="reference internal" href="../appendix/specdef_values.html">Example macro implementations</a></li>
<li class="toctree-l1"><a class="reference internal" href="../appendix/history.html">Changes to the API</a></li>
</ul>
<ul>
<li class="toctree-l1"><a class="reference internal" href="../psa_c-identifiers.html">Index of API elements</a></li>
</ul>
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